Wireless communication devices (such as cellular phones) use power amplifiers to amplify and transmit radio frequency (RF) signals. These RF signals must be transmitted at a prescribed power level despite varying environmental conditions such as temperature, supply voltage, load impedance, etc. Generally, the output of a power amplifier should be maintained at the prescribed power level, since excessive power levels needlessly drain a battery supply and may also violate health and safety requirements.
FIG. 1 shows a prior art RF amplifier circuit 11. A power amplifier 13 amplifies an RF input signal 14 and generates an output signal 17 at a prescribed power level. A directional coupler 15 directs a portion (“coupled signal 18”) of the output signal 17 and sends the coupled signal 18 to a detector 19. The detector 19 detects the peak voltage Vpk of the coupled signal 18 and applies it to a comparator 21. The comparator 21 compares Vpk with a reference voltage Vref, where Vref typically represents the prescribed power level required by the operating protocol. The output of the comparator 21 is a gain control signal 23 that adjusts the gain of the power amplifier 13.
The prior art RF amplifier circuit 11 has several disadvantages. For example, the drive level into output stage of the power amplifier 13 is not limited. Under certain load conditions, the power amplifier 13 has poor linearity that can cause RF spectrum failures, or unnecessary current drain. Also, the directional coupler 15 is connected at the output of the amplifier circuit 11, diverting part of the output signal 17 to the detector 19. This reduces the power of the transmitted output signal 17, which means that the overall system efficiency suffers, and the power amplifier 13 must be designed to amplify with greater power to account for the drain on the signal by the directional coupler 15. Furthermore, the directional coupler is typically a surface mount component that is expensive and takes up a relatively large area. Therefore, there remains a need for an improved RF amplifier circuit.